Microwave induced destruction of siloxanes and hydrogen sulfide in biogas

ABSTRACT

The invention is an apparatus and method to remove hydrogen sulfide and siloxanes from biogas and destroy the contaminants in microwave reactors. Hydrogen sulfide and siloxane are removed from biogas using an adsorbent media such as activated carbon. The media is regenerated in a microwave reactor where the hydrogen sulfide and siloxane are removed in a sweep gas. In one process, siloxane is oxidized to silicon dioxide in a second microwave reactor and removed with a filter. Hydrogen sulfide if first oxidized to sulfur dioxide, then reduced to sulfur in a third microwave reactor and removed with a filter. In another process, siloxane is combined with water to form silicon dioxide and hydrogen sulfide is reduced to elemental sulfur in a microwave reactor. These reactants are removed with a filter. The remaining sweep gas containing hydrogen and low molecular weight hydrocarbons is returned to the biogas stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional application Ser.No. 60/884,983 filed on Jan. 15, 2007, incorporated herein by referencein its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. §1.14.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to destruction of contaminants inbiogas, and more particularly to microwave induced destruction ofsiloxanes and hydrogen sulfide contaminants from biogas.

2. Description of Related Art

Developing biogas resources for electric generation is challengingbecause of pretreatment requirements to accommodate the generatingequipment and combustion controls or post treatment systems needed tomeet increasingly stringent emission requirements, especially inCalifornia. Removing siloxanes and hydrogen sulfide (H₂S) from biogas iscrucial because their combustion products increase engine maintenanceintervals and interfere with existing post-combustion nitrogen oxides(NOx), sulfur oxides (SOx), and hydrocarbon removal technologies thatare required to meet regional air quality standards.

Siloxanes are a family of man-made organic compounds that containsilicone, oxygen and methyl groups. As a consequence of their widespreaduse in consumer products, siloxanes are found in wastewater and in solidwaste deposited in landfills. At wastewater treatment plants andlandfills, low molecular weight siloxanes volatilize into digester andlandfill gas. When this biogas is combusted in gas turbines, boilers, orinternal combustion engines, siloxanes are converted to silicon dioxide(SiO₂) and micro-crystalline quartz, which can deposit in the combustionand/or exhaust stages of the equipment as an abrasive white powder,contributing to engine wear and component failure. Landfills havesiloxane concentrations on average from as low as 0.2 mg/m³ to about 10mg/m³ but can be as high as about 140 mg/m³. Siloxane concentrations canvary greatly in landfills as they age whereas the concentration inwastewater digesters is fairly constant. Animal waste digesterstypically contain little or no siloxane unless offsite waste material isadded. Manufacturers of combustion turbines and reciprocating engineshave begun limiting feed gas siloxane concentrations to between 5-28mg/m³ for internal combustion (IC) engines, and 0.1 to 0.03 mg/m³ forgas turbines.

Table 1 presents common volatile siloxanes found in biogas with theirmolecular weight, vapor pressure, boiling point, chemical formula, andwater solubility. Abbreviations are commonly used to identify thesiloxane compounds. Siloxanes that are cyclic in structure have a singleabbreviation of D. Siloxanes that have a linear structure have twoabbreviations using an L or M nomenclature.

Additional organosilicon compounds such as Trimethylsilanol (Si(CH₃)₃OH)and Tetramethylsilane (Si(CH₃)₄) may also be present in biogas and areincluded in the term siloxane for purposes of describing this invention.

Both wet and dry scrubbers have been used to remove siloxanes frombiogas. The major disadvantages of wet scrubbers are the production ofhazardous liquid waste and the fact that complete silicon elimination isdifficult to obtain since the highly volatile siloxanes are strippedfrom the solvent at elevated gas flow rates.

Cyclic and linear (dimethyl) siloxanes are very stable against chemicaland biochemical degradation. However, strong acids or bases catalyze thecleavage of Si—O bonds to produce poly dimethyl siloxanes. Due to thehigh content of CO₂ in biogases, the application of caustic absorbentsfor siloxane removal is not practical due to carbonate formation. Usingacidic solutions is also difficult due to the hazardous liquid wastes.

The most common adsorbent used in dry scrubbing is Granular ActivatedCarbon (GAC), because it is cheaper than alternative adsorbents such asmolecular sieves and polymer beads. GAC adsorbs siloxanes, H₂S, heavyhydrocarbons and organic halides. Since siloxanes are difficult todesorb from GAC, the adsorbent must be replaced regularly. The siloxanesaturated GAC is typically burned as fuel or disposed of in a landfillwhere the volatile compounds, including siloxanes, can reappear issubsequent landfill gas. Other contaminants, such as H₂S, compete withsiloxanes for adsorption sites on the GAC. Temperature and water contentof the biogas also affects GAC adsorption capacity. Different siloxanecompounds will also exhibit different adsorption capacities, generallywith adsorption capacity increasing with increasing molecular weight.

Chilling biogas down to 40 deg F. is used to dry the gas for turbinesand also removes siloxane in a range of 15% to about 50%. A proprietaryprocess that refrigerates biogas to about −20 deg F. claims 95-99%siloxane removal rates. Both refrigeration processes use a GAC adsorberfor final siloxane removal. Refrigeration uses significant energy toclean the biogas.

Consequently, GAC is currently the most practical and economic adsorbentfor the removal of siloxanes in biogas. A better reactivation technologyto desorb high molecular weight siloxanes can remove the need to chillthe biogas and reduce the major lifecycle cost of GAC systems, which iscarbon replacement. As discussed later, microwave energy can easilyremove high molecular weight siloxanes from GAC.

Anaerobic digesters produce about 50%-60% methane, 40%-50% CO₂ andsulfur impurities mostly in the form of H₂S. Landfill gas typically haslower methane concentrations and added hydrocarbons derived primarilyfrom solvents in the landfill. Landfill gas and digester gas may alsocontain a variety of trace compounds. More than 140 substances have beenidentified so far and they reach a total concentration of up to 2000mg/m³ (0.15% by volume). During the combustion process, H₂S andhalogenated compounds in biogas form corrosive acids including H₂SO₄ andHCl.

Hydrogen Sulfide has a strong odor that can be detected at thresholdlevels of about 0.47 parts per billion (ppb) and has an OSHA IDLH levelof 300 parts per million (ppm). Digesters have H₂S levels of about 25ppm to over 1,000 ppm for animal digesters, where landfills gas levelsusually vary from 10 ppm to over 100 ppm. Assuming emissions of SOx arenot an issue, boilers can tolerate H₂S, levels up to 1,000 ppm,reciprocating engines about 10 to 100 ppm and fuel cells 10 ppm to 20ppm.

Reciprocating engines operating on digester biogas compared to naturalgas engines cost about 20% more to install and about 80% more tomaintain. Sulfur plugs filters, causes deposits on valves and cylindersand contaminates lubricating oil. It has been reported that someoperators must change spark plugs frequently ($1,000 annually) andchange oil as often as weekly ($350 to $1,000 per month).

The H₂S pretreatment system of choice for digesters with 100 to 1,000+ppm H₂S has been gas contact with an iron oxide media. The most wellknown treatment system is an iron sponge. This is a container of ironoxide impregnated media (typically woodchips) that scrubs the inlet gasfrom the digester. The iron sponge is sized for a residence time ofabout 60 seconds and, the media can collect up to about 2.5 times itsweight in sulfur compounds. The media can be partially regenerated byexposure to air or by wetting for about 10 days. Eventually the mediamust be discarded and replaced with new media. With increasingfrequency, the spent media is classified a hazardous waste by localregulators. One example of an iron sponge system costs about $50,000 toinstall with annual operating costs ranging from $250 to $4,000.

Proprietary iron-oxide media such as SulfaTreat®, Sulfur-Rite®, andMedia-G2® have been installed as improved alternatives to the ironsponge at a few digester sites. These use different media and additionalchemical treatment to remove sulfur. Some of these media have limitedregeneration capacity or can be safely deposited in a landfill. Onedairy digester site using Media-G2 has two vessels with about 760 kg ofmedia each with a residence time of about 62 seconds per vessel. Annualmedia consumption ranges from 1,460 kg to about 5,900 kg with mediareplacement costs on the order of $2,050 to $8,290.

GAC and other carbon products are used extensively for filtration ofcontaminants in water and gas streams. GAC contains micro-pores thatcapture and hold many organic and polar molecules and is more effectivefor larger molecules. In other cases, the carbon acts as a catalyst todrive a reaction with the carbon and the selected molecule in a processknown as chemisorption.

Commercially available GAC and Pelletized Activation Carbon (PAC) havethe surface area in the range of 800-1000 m²/g. These activated carbonseasily adsorb SO₂, NOx, and VOCs. The carbon adsorption capacity isdependent on the composition of gas. GAC and PAC also adsorb siloxanesand H₂S in biogas.

GAC adsorbs most VOCs and is used in removing common solvent vapors usedin drying cleaning and parts washing operations. The carbon adsorptioncapacity is strongly dependent on the VOC molecular weight. Theadsorption capacities of toluene and methylene chloride at the roomtemperature are 20 and 5 g/100 gGAC, respectively. However theadsorption capacity of CH₄ in GAC is negligible.

The GAC adsorption capacity for H₂S is 5-15% by weight depending onloading of water and other contaminants. Therefore, GAC can be usedeconomically to remove the H₂S from biogas that contains lowerconcentrations of H₂S. Typically, used GAC is disposed of in a landfillwhen saturated with H₂S.

Impregnating GAC with alkaline or oxide solids enhance the physicaladsorptive characteristics of the carbon with chemical reaction. Sodiumhydroxide (NaOH), sodium carbonate (Na₂CO₃), and potassium hydroxide(KOH) are common impregnators. The metal oxide impregnation increasesthe GAC adsorption capacity significantly-especially if a small amountof oxygen is present in the biogas stream. Typically, 20-25% loading byweight of H₂S can be achieved. The metal-impregnated GAC is almost twicemore expensive than GAC. However, the use of metal-impregnated GAC willbe more economical for the adsorbers without the on-site carbonreactivation because of its greater adsorption capacity. If the on-siteGAC regeneration is available, the use of regular GAC for H₂S removal ispreferable to the metal oxide-impregnated GAC.

The GAC adsorption capacity for Siloxanes has been reported to be from 1to 1.5 percent by weight. This capacity is affected by the species ofSiloxane in the gas, other contaminants in the gas including H₂S, andthe temperature and water content of the gas.

Once GAC can no longer adsorb a chemical compound, breakthrough willoccur where the compound will flow all the way through the bed withoutbeing adsorbed. At this point, the GAC is no longer effective and mustbe replaced. In many cases, such as GAC filled water filters orrespirators, the GAC is thrown away and a fresh GAC filter or cartridgeis installed. In large scale processes, or where the contaminant can berecovered or destroyed, regeneration of the GAC may be preferred.

There are four processes commonly used for GAC regeneration: TemperatureSwing Adsorption (TSA), Pressure Swing Adsorption (PSA) Inert Purge andDisplacement Purge. TSA takes place by heating the GAC to removecontaminants. With PSA the adsorption takes place at an elevatedpressure and regeneration at a lower pressure. Inert gas purge reducesthe partial pressure of the adsorbate in the gas phase so thatdesorption occurs. A purge gas that is more strongly adsorbed than thecontaminant is used to desorb the original contaminant. Steamregeneration is a combination of TSA and purge. In each process, thecontaminant is still present in the purge stream and must be captured,burned or vented to the atmosphere.

High molecular weight siloxane compounds make conventional thermalregeneration difficult due to their low vapor pressures. Heavy moleculesaccumulate in GAC after thermal regeneration, limiting the number ofactivation cycles carbon can by subject to before its performance isinadequate.

A molecular sieve is a material containing tiny pores of a precise anduniform size that is used as an absorbent for gases and liquids.Molecules small enough to pass through the pores are absorbed whilelarger molecules are not. It is different from a common filter in thatit operates on a molecular level. For instance, a water molecule may besmall enough to pass through while larger molecules are not. Because ofthis, they often function as a desiccant. A molecular sieve can absorbwater up to 22% of its own weight so removal of water in biogas isimportant. Often they consist of aluminosilicate minerals, clays, porousglasses, microporous charcoals, zeolites, active carbons, silica gel orsynthetic compounds that have open structures through which smallmolecules, such as nitrogen and water can diffuse. They are classifiedby pore size such as type 3 A or type 4 A designating pore size inangstroms. Traditional methods for regeneration of molecular sievesinclude pressure change, as in oxygen concentrators, or by heating andpurging with a carrier gas. Molecular sieve materials can also beregenerated in a microwave system.

Quantum radiofrequency (RF) physics is based upon the phenomenon ofresonant interaction with matter of electromagnetic radiation in themicrowave and RF regions since every atom or molecule can absorb, andthus radiate, electromagnetic waves of various wavelengths. Therotational and vibrational frequencies of the electrons represent themost important frequency range. The electromagnetic frequency spectrumis usually divided into ultrasonic, microwave, and optical regions. Themicrowave region is from 300 megahertz (MHz) to 300 gigahertz (GHz) andencompasses frequencies used for much communication equipment. Forinstance, refer to Cook, Microwave Principles and Systems,Prentice-Hall, 1986.

Often the term microwaves or microwave energy is applied to a broadrange of radiofrequency energies particularly with respect to the commonheating frequencies, 915 MHz and 2450 MHz. The former is often employedin industrial heating applications while the latter is the frequency ofthe common household microwave oven and therefore represents a goodfrequency to excite water molecules. In this writing the term“microwave” or “microwaves” is generally employed to represent“radiofrequency energies selected from the range of about 500 to 5000MHz”, since in a practical sense this large range is employable for thesubject invention.

The absorption of microwaves by the energy bands, particularly thevibrational energy levels, of atoms or molecules results in the thermalactivation of the nonplasma material and the excitation of valenceelectrons. The nonplasma nature of these interactions is important for aseparate and distinct form of heating employs plasma formed by arcconditions at a high temperature, often more than 3000.degree. F., andat much reduced pressures or vacuum conditions. For instance, refer toKirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition,Supplementary Volume, pages 599-608, Plasma Technology. In microwavetechnology, as applied in the subject invention, neither of theseconditions is present and therefore no plasmas are formed.

Microwaves lower the effective activation energy required for desirablechemical reactions since they can act locally on a microscopic scale byexciting electrons of a group of specific atoms in contrast to normalglobal heating which raises the bulk temperature. Further thismicroscopic interaction is favored by polar molecules whose electronsbecome easily locally excited leading to high chemical activity;however, nonpolar molecules adjacent to such polar molecules are alsoaffected but at a reduced extent. An example is the heating of polarwater molecules in a common household microwave oven where the containeris of nonpolar material, that is, microwave-passing, and staysrelatively cool.

In this sense microwaves are often referred to as a form of catalysiswhen applied to chemical reaction rates; thus, in this writing the term“microwave catalysis” refers to “the absorption of microwave energy bycarbonaceous materials when a simultaneous chemical reaction isoccurring” For instance, refer to Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd Edition, Volume 15, pages 494-517, Microwave Technology.

BRIEF SUMMARY OF THE INVENTION

The features of the present invention include (1) removal of siloxanes,H₂S and hydrocarbons from biogas with adsorbent media, (2) microwavereactivation of adsorbent media containing siloxanes and H₂S, (3)microwave-induced oxidation of siloxanes and H₂S in the SiC catalystbed, and (4) microwave-induced reduction reaction of SO₂ with GAC toproduce elemental sulfur and carbon dioxide. The media adsorberconcentrates siloxanes and H₂S from biofuel gas into the media, whichleads to a small, concentrated contaminant stream during microwaveregeneration.

High molecular weight siloxane compounds make conventional thermalregeneration difficult due to their low vapor pressures. Sincemicrowaves penetrate and volumetrically heat GAC particles, heavysiloxane molecules may be de-polymerized into lighter, more easilydesorbed, compounds. This ability is a major advantage over thermalcarbon regeneration, because heavy molecules accumulate in GAC afterthermal regeneration, limiting the number of activation cycles carboncan by subject to before its performance is inadequate.

The SiC catalyst is an excellent microwave absorber and acts as anoxidizing catalyst in the microwave field. Consequently, siloxanes andH₂S in the sweep gas from the microwave regeneration reactor can beoxidized at lower temperatures than in a conventional catalyst bed. Themicrowave-induced oxidation starts at room temperature and the SiC bedtemperature increases rapidly when microwaves are applied. Theefficiency of the microwave SiC bed oxidization is greater than 99% forhydrocarbons in air.

Subsequently, the SO₂ reacts with carbon as soon as microwave energy isapplied to a GAC bed to produce carbon dioxide and elemental sulfur thatcan be captured by a filter. Consequently, no secondary air pollutantsare produced from the microwave biogas treatment process.

The unique characteristics of microwave energy are utilized tosignificantly enhance chemical reactions and solvent desorption fromsaturated carbon. A microwave regeneration and treatment system has beenconstructed that can regenerate GAC at costs ranging from $0.20 to $0.60per pound. The regeneration system can be scaled from a few pounds perhour of GAC regenerated at the biogas source to 500 pounds per hour ormore for a large or regional treatment facility.

The GAC is an excellent microwave absorbent and its temperatureincreases rapidly when exposed to microwaves. The siloxanes adsorbedonto the GAC will be de-polymerized into smaller molecules during themicrowave regeneration. Consequently, siloxanes desorb easily bymicrowave energy and GAC recovers its original adsorption capacity. Forcomparison, the rate of microwave solvent desorption is an order ofmagnitude greater than the conventional thermal desorption rate.

A two-stage adsorption system is used to remove siloxanes and H₂S fromthe biogas simultaneously. The adsorber system is preceded by a waterseparator and equipped with the inlet and outlet valves that can easilychange the flow direction. A two stage radial adsorber is preferred forhigher flow rates at lower backpressure.

The biogas typically flows into a water knockout pot or a refrigeratedcondenser to remove free water in the biogas prior to entering into theadsorption system. The water free gas enters the adsorber containing theoldest media (first-stage or primary adsorber). Note that the GAC mediaadsorption capacity for siloxanes increases with decreasing moisturecontent in biogas. The gas leaving the first-stage adsorber flows intothe adsorber containing fresh or regenerated media (second-stage orsecondary adsorber). Consequently, whenever the saturated media isremoved from the first-stage adsorber, the inlet and outlet valves arereversed to change the gas flow direction. As a result, the first-stageadsorber containing regenerated media becomes the second-stage adsorber.

The GAC adsorption capacity for H₂S is about 5-15% by weight. If 3,000lbs GAC adsorbers are used, GAC in the first-stage adsorber needs to bechanged every 100 days for a 100 kW biogas power generating facility for500 ppm H₂S. For a 400 kW biogas generator, the GAC in the first-stageadsorber needs to be changed every 25 days. This carbon change-outschedule is much shorter for digesters producing H₂S concentrationgreater than 500-ppm.

As shown in Table 1, the molecular weight of siloxanes is much greaterthan the MW of H₂S and vapor pressure of siloxanes is much lower thanH₂S. The adsorption capacity of GAC for siloxane is about one tenth thatof H₂S, but the concentration of siloxane is also a fraction of the H₂Sconcentration. Consequently, the presence of siloxanes should notincrease the GAC change-out cycle significantly.

The two-stage media adsorption system should remove 99.9% of thesiloxanes and H₂S from the biogas. For biogases containing H₂Sconcentration greater than 500 ppm, an iron sponge or other commerciallyavailable H₂S removal system should be installed prior to the GACadsorber system. The H₂S removal efficiency of the conventional ironsponge system is about 80%.

An embodiment of the invention is an apparatus for removing siloxaneadsorbed in adsorbent media and decomposing the siloxane that comprisesa first microwave reactor having a source of microwave energy, where thefirst microwave reactor is configured to receive adsorbent mediacontaining siloxane, where siloxane is removed from the adsorbent mediawhen exposed to microwave energy in the first microwave reactor, meansfor forming silicon dioxide from siloxane fluidly connected to the firstmicrowave reactor, and a source of sweep gas connected to the firstmicrowave reactor, where sweep gas flows from the sweep gas sourcethrough the first microwave reactor and through the means for formingsilicon dioxide.

An aspect of the invention is where the adsorbent media containshydrogen sulfide, where hydrogen sulfide is removed from the adsorbentmedia when exposed to microwave energy in the first microwave reactor,where hydrogen sulfide is transported by the sweep gas from the firstmicrowave reactor to the means for forming silicon dioxide, and whereelemental sulfur is formed from hydrogen sulfide in the means forforming silicon dioxide.

Another aspect of the invention is a biogas adsorber fluidly connectedto the first microwave reactor, where the biogas adsorber is configuredto contain the adsorbent media, means for transporting the adsorbentmedia from the biogas adsorber to the first microwave reactor, aparticulate filter fluidly connected to the means for forming silicondioxide, where biogas containing siloxane is passed through theadsorbent media in the biogas adsorber, and where the sweep gas flowsfrom the means for forming silicon dioxide through the filter and to thebiogas adsorber.

A further aspect of the invention is where the biogas contains heavyhydrocarbons having a molecular weight greater than forty five, whereheavy hydrocarbons are adsorbed from the biogas in the adsorbent media,where heavy hydrocarbons are removed from the adsorbent media whenexposed to microwave energy in the first microwave reactor, where heavyhydrocarbons are transported by the sweep gas from the first microwavereactor to the means for forming silicon dioxide, where methane isformed from heavy hydrocarbons in the means for forming silicon dioxide,and where methane is transported by the sweep gas from the means forforming silicon dioxide to the biogas adsorber.

A still further aspect of the invention is a second microwave reactorfluidly coupled to the first microwave reactor, a reducing agentpositioned in the second microwave reactor, a source of microwave energycoupled to the second microwave reactor, and a source of water fluidlyconnected to the second microwave reactor, where silicon dioxide isformed when water is combined with siloxane and exposed to microwaveenergy in the second reactor.

Another aspect of the invention is where the reducing agent comprisescarbon.

A further aspect of the invention is a second microwave reactor fluidlyconnected to the first microwave reactor, an oxidation catalystpositioned in the second microwave reactor, a source of microwave energycoupled to the second microwave reactor, and a source of oxygen fluidlyconnected to the second microwave reactor, where silicon dioxide isformed when oxygen is combined with siloxane and exposed to microwaveenergy in the second reactor.

A yet further aspect of the invention is a first means for filteringfluidly connected to the second microwave reactor, a third microwavereactor fluidly connected to the first means for filtering, where thethird microwave reactor has a source of microwave energy, a reducingagent positioned in the third microwave reactor, and a second means forfiltering, the second means for filtering fluidly connected to the thirdmicrowave reactor, where the sweep gas flows from the second microwavereactor, through the first filtering means, through the third microwavereactor and through the second filtering means.

Another aspect of the invention is where the adsorbent media containshydrogen sulfide, where hydrogen sulfide is transported by the sweep gasfrom the adsorbent media in the first microwave reactor to the secondmicrowave reactor, where hydrogen sulfide is combined with oxygen whenexposed to microwave energy in the second microwave reactor to formsulfur dioxide, where sulfur dioxide is transported to the thirdmicrowave reactor by the sweep gas, where hydrogen sulfide is reduced toelemental sulfur in the third microwave reactor when exposed tomicrowave energy, and where elemental sulfur is removed from the sweepgas by the second filtering means.

A further aspect of the invention is where the adsorbent media comprisescarbon, where the oxidation catalyst comprises silicon carbide, andwhere the reducing agent comprises carbon.

A still further aspect of the invention is where the reducing agentfurther comprises a metal oxide.

Another embodiment of the invention is an apparatus for removingsiloxane, and hydrogen sulfide adsorbed in adsorbent media anddecomposing the siloxane and hydrogen sulfide that comprises a firstmicrowave reactor having a source of microwave energy, where the firstmicrowave reactor is configured to receive adsorbent media containingsiloxane and hydrogen sulfide, a second microwave reactor fluidlyconnected to the first microwave reactor, the second microwave reactorhaving a source of microwave energy, a first particulate filter fluidlyconnected to the second microwave reactor, where the first filter isconfigured to remove particulate silicon dioxide and sulfur, and asource of sweep gas connected to the first microwave reactor, wheresweep gas flows from the sweep gas source through the first microwavereactor, through the second reactor and through the first filter.

Another aspect of the invention is where the adsorbent media ispositioned in the first microwave reactor, where siloxane and hydrogensulfide are removed from the adsorbent media when exposed to microwaveenergy, where siloxane and hydrogen sulfide are transported by the sweepgas from the first microwave reactor to the second microwave reactor,where water is introduced into the second microwave reactor, wheresilicon dioxide is formed in the second microwave reactor when siloxaneand water are exposed to microwave energy, where silicon dioxide isremoved from the sweep gas in the first filter, where sulfur is formedin the second microwave reactor when hydrogen sulfide is exposed tomicrowave energy, and where sulfur is removed from the sweep gas in thefirst filter.

A further aspect of the invention is a biogas adsorber fluidly connectedto the first microwave reactor, where the adsorbent media is positionedin the biogas adsorber, where biogas containing siloxane and hydrogensulfide is passed through the adsorbent media in the biogas adsorber,where the adsorbent media is transported to the first microwave reactor,and where the sweep gas flows from the first filter to the biogasadsorber.

A still further aspect of the invention is an oxidation catalystpositioned in the second microwave reactor, a third microwave reactorfluidly connected to the first filter, where the third microwave reactorhas a source of microwave energy, a reducing agent positioned in thethird microwave reactor, a second filter fluidly connected to the thirdmicrowave reactor, where the second filter is configured to removesulfur from the sweep gas, and a source of oxygen connected to thesecond reactor, where the sweep gas flows from the first filter, throughthe third microwave reactor and through the second filter.

A further embodiment of the invention is a method for removing siloxanefrom adsorbent media and decomposing the siloxane that comprisesproviding a first microwave reactor having a source of microwave energy,where the first microwave reactor is configured to receive adsorbentmedia containing siloxane, providing a second microwave reactor having asource of microwave energy fluidly connected to the first microwavereactor, providing a sweep gas flowing through the first microwavereactor and the second microwave reactor, positioning adsorbent mediacontaining siloxane in the first microwave reactor, applying microwaveenergy to the first microwave reactor to remove siloxane from theadsorbent media, transporting siloxane to the second reactor by thesweep gas, introducing water into the second reactor, and applyingmicrowave energy to the second microwave reactor to form silicon dioxidefrom siloxane.

Another aspect of the invention is where the second microwave reactorcontains a reducing agent.

A further aspect of the invention is where the adsorbent media containshydrogen sulfide, applying microwave energy to the first microwavereactor to remove hydrogen sulfide from the adsorbent media,transporting hydrogen sulfide in the sweep gas from the first microwavereactor to the second microwave reactor, and forming elemental sulfurfrom hydrogen sulfide when exposed to microwave energy in the secondmicrowave reactor.

A still further aspect of the invention is providing a first particulatefilter fluidly connected to the second microwave reactor, providing anadsorber fluidly connected to the first microwave reactor, positioningthe adsorbent media in the adsorber, flowing biogas containing siloxanethrough the adsorbent media in the adsorber, adsorbing siloxane from thebiogas into the adsorbent media, transporting the adsorbent mediacontaining siloxane to the first microwave reactor, flowing the sweepgas from second microwave reactor, through the first filter and into theadsorbent media in the adsorber, and removing silicon dioxide from thesweep gas in the first filter.

Another aspect of the invention is where the biogas contains heavyhydrocarbons having molecular weight greater than forty five, adsorbingheavy hydrocarbons from the biogas into the adsorbent media,transporting the adsorbent media to the first microwave reactor,applying microwave energy to the first microwave reactor to remove heavyhydrocarbons from the adsorbent media, transporting heavy hydrocarbonsin the sweep gas from the first microwave reactor to the secondmicrowave reactor, forming methane from heavy hydrocarbons when exposedto microwave energy in the second microwave reactor, and transportingmethane from the second microwave reactor to the adsorber in the sweepgas.

A further aspect of the invention is providing an oxidizing catalyst inthe second microwave reactor, providing a source of oxygen to the secondmicrowave reactor, providing a first particulate filter fluidlyconnected to the second microwave reactor, providing a third microwavereactor having a source of microwave energy, where the third microwavereactor is fluidly connected to the second microwave reactor, providinga reducing agent in the third microwave reactor, providing a secondparticulate filter fluidly connected to the third microwave reactor,where the sweep gas flows from the second microwave reactor through thefirst filter, through the third microwave reactor and through the secondfilter, where the adsorbent media contains hydrogen sulfide, applyingmicrowave energy to the first microwave reactor to remove hydrogensulfide from the adsorbent media, transporting siloxane and hydrogensulfide in the sweep gas from the first microwave reactor to the secondmicrowave reactor, forming silicon dioxide from siloxane when exposed tomicrowave energy in the second microwave reactor, removing silicondioxide in the first filter, forming sulfur from hydrogen sulfide whenexposed to microwave energy in the second microwave reactor,transporting sulfur dioxide in the sweep gas from the second microwavereactor to the third microwave reactor, forming sulfur from sulfurdioxide when exposed to microwave energy in the third microwave reactor,and removing sulfur from the sweep gas in the second filter.

Another aspect of the invention is where the adsorbent media is carbon,where the oxidizing catalyst is silicon carbide, and where the reducingagent is carbon.

A further aspect of the invention is providing a metal oxide in thereducing agent.

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic view of a process to remove siloxane and hydrogensulfide from biogas using media adsorption.

FIG. 2 is a schematic view of an apparatus for removing siloxane and H₂Sfrom media with a microwave regeneration system and forming inertcomponents from the contaminants.

FIG. 3 is a schematic view of another embodiment for removing siloxaneand hydrogen sulfide from media and filtering solids.

FIG. 4 is a schematic view of a microwave reactor used in the apparatusin FIG. 2 and FIG. 3.

FIG. 5 is a schematic view of a system to remove siloxane and hydrogensulfide from biogas without atmospheric emissions.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the apparatus generally shown inFIG. 1 through FIG. 5. It will be appreciated that the apparatus mayvary as to configuration and as to details of the parts, and that themethod may vary as to the specific steps and sequence, without departingfrom the basic concepts as disclosed herein.

FIG. 1 is a schematic view of a process 10 for removing siloxane andhydrogen sulfide (H₂S) contaminants from biogas. Biogas is primarilymethane generated by bacteria in anaerobic conditions such as a digesteror landfill 12. Raw biogas flows from the digester to a water knock out13, such as a chiller system, to remove excess water 14. The biogas thenflows to media adsorption vessels 15, 16, and 17 also called adsorbers.

Media adsorbers 15, 16, 17, contain an absorbent media, such asGranulated Activated Carbon (GAC), and remove H₂S and siloxanes from thebiogas through a chemisorption process. Some water present in the biogasis also adsorbed by the GAC. Typically, two adsorbers are in use at atime so that one can be serviced while the other is in operation. Insome systems the flow of biogas can be reversed between any two vessels.Valves and controls are omitted for clarity.

The cleaned biogas flows from adsorbers 15, 16, 17, to an engine,turbine or boiler 18 to make heat, steam and/or electricity.Alternatively, the cleaned biogas is flared. In a typical operation, themedia in one of the adsorbers 15, 16, 17, is removed when breakthroughof H₂S or siloxanes occurs and is replaced with fresh or regeneratedmedia. In one embodiment, adsorbers 15, 16, and 17 are portable and canbe removed from the system and transported as a canister containingmedia.

If hydrogen sulfide levels are higher than about 500 ppm in the biogas,a sulfur removal system such as an iron sponge (not shown) may be usedto reduce the high level of sulfur prior to adsorbers 15, 16, and 17.

In one embodiment of the invention, GAC in adsorbers 15, 16 and/or 17 isimpregnated with a metal oxide, such as sodium hydroxide to increaseadsorption capacity for H₂S and siloxanes.

In another embodiment of this invention, each adsorber 15, 16, and 17contain media selected for a specific chemical property or pore sizethat corresponds to a specific range of contaminant molecular weights.For example, adsorber 15 may have media with larger pores (2.5 to 3.5nm) for siloxanes with a molecular weight over 250, and adsorber 17 mayhave media with smaller pores (0.7-2.5 nm) for siloxanes or contaminantswith a molecular weight of 100 or less. By sequencing or layering thesemedia in the order of largest pore structure down to the smallest,removal efficiencies of 50% greater than a homogenous bed of media canbe realized before breakthrough of a siloxane species. A select mediacan also be specified for a particular compound found in a biogassource. The media in each adsorber can be regenerated separate from theothers to maintain the media properties in each adsorber.

Media such as GAC, pelletized activated carbon (PAC), zeolites,molecular sieve and silica gel may be specified for each adsorberdepending on the composition of the contaminants in the biogas and thedesired pore characteristics in each adsorber.

Media which will adsorb siloxane, hydrogen sulfide and/or water andwhich will absorb microwave energy are preferred. Media that willregenerate the pore sites when exposed to microwave energy are mostpreferred.

Table 2 is an example of a material balance for a system with a flowrate of 100 SCFM with a composition of 55% methane, 44.9% CO₂, and 50ppmv H₂S, and trace siloxanes. Here, four media adsorbers are used. Theperformance of four media, each with different pore sizes from larger tosmaller, are estimated to complete the material balance.

The removal performance of the example media is presented in Table 3.The multi-stage adsorption system is expected to remove at least 90% ofthe siloxanes and H₂S present in the biogas.

FIG. 2 illustrates a process flow diagram for a microwave regenerationsystem 50 for regenerating the saturated media from vessels 15, 16, and17 shown in FIG. 1. Valves and controls have been omitted for clarity.If the microwave regeneration system is not co-located with theadsorbers 15, 16, 17, the media saturated with siloxane, H₂S and wateris transported to the microwave regeneration facility in bulk or inportable adsorption vessels. The contaminated media is stored in themedia feed tank 22.

A first microwave reactor 23 has a source of microwave energy 24 and avertical tube 25 that is transparent to microwave energy. In oneembodiment, vertical tube 25 is made of quartz glass.

The saturated media from media hopper 22 is placed in vertical tube 25in microwave reactor 23. The media is exposed to microwave energy intube 25. It is preferable that tube 25 be sealed from air (oxygen) whenexposed to microwave energy to prevent oxidizing the GAC. Aftermicrowave treatment, media is removed from tube 25 and transported tothe regenerated media container 26 where it can be reused in adsorbers15, 16 and 17.

A low volume, inert, sweep gas, such as Nitrogen 27, flows in tube 28 ina countercurrent direction through tube 25 in first microwave reactor23. The siloxanes and H₂S, as well as water and high molecular weighthydrocarbons are desorbed rapidly from the media when exposed tomicrowaves. Some siloxanes may be de-polymerized at the carbon surfaceby microwave energy to produce lower molecular weight siloxanes beforedesorbing from the media. As a result of the exposure to microwaveenergy, no siloxane, water or hydrocarbon components will remain in themedia. The power level of the microwave, the duration of media exposure,and flow rate of sweep gas are controlled to remove the siloxanecomponents from media but not decompose the siloxanes into SiO₂. Theactive pores of the media are also regenerated by exposure to themicrowave energy in this step.

The media can be regenerated in microwave reactor 23 in batch mode orcontinuous flow mode.

Siloxanes, water vapor and any VOCs desorbed from the media aretransported out of microwave reactor 23 by the sweep gas in tube 29 to areduction microwave reactor 30 having a source of microwave energy 31and a vertical tube 32 that is transparent to microwave energy. Tube 32contains a reducing agent such as GAC.

The GAC is an excellent microwave absorbent and also reducing agent forhydrocarbons. Consequently, the siloxanes will react with water in thepresence of the GAC bed when exposed to microwave energy to producemethane and SiO₂ as shown in two examples below:Si₂(CH₃)₆O(Hexamethyldisiloxane)+3H₂O→6CH₄+2SiO₂Si₅(CH₃)₁₀O₅(Decamethylcyclopentasiloxane)+5H₂O→10CH₄+5SiO₂

Other siloxanes species and siloxane containing hydrocarbons react withwater when exposed to microwave energy in the reduction reactorfollowing a similar reaction path as shown above. If the media fromadsorber 15, 16, 17 is unusually dry, water can be introduced into sweepgas in tube 29 and into reduction reactor 30.

If hydrogen sulfide is present in the biogas, it will be adsorbed in themedia in adsorbers 15, 16, 17 and desorbed from the media in microwavereactor 23 in the presence of microwave energy and removed in sweep gasin tube 29. Biogas from a landfill may have 1-2% oxygen. Some media,such as GAC, will adsorb a fraction of this oxygen. If oxygen is presentin the sweep gas or in the GAC, hydrogen sulfide will preferentiallyform water in reactor 30:H₂S+½O₂→H₂O+S

When all oxygen is removed or consumed, hydrogen sulfide is dissociatedinto hydrogen and elemental sulfur in the presence of the reducing agentwhen exposed to microwave energy in reduction reactor 30 as shown below:H₂S→H₂+S

Any high molecular weight VOCs containing Oxygen will decompose to formCO₂ and lower molecular weight hydrocarbons such as methane in reductionreactor 30. Other hydrocarbons will crack to lower molecular weighthydrocarbons, such as methane, and release hydrogen in the presence ofmicrowave energy. The reactants then exit reactor 30 at outlet 34 fordisposal. If halogenated hydrocarbons are present, the GAC in thereduction reactor 30 can be impregnated with a metal oxide such as NaOH.The sodium will react with the halogens to produce halogen salts, suchas NaCl, that will be removed as a particulate at outlet 34. Ash fromreacted GAC is also removed at outlet 34.

The volume of reactants at outlet 34 is very small compared to thevolume of biogas treated. The reactants can be further treated byfiltering the particulates or passing the remaining gasses through aflare if desired.

In one embodiment, reactor 23 and reactor 30 share the same source ofmicrowave energy. In another embodiment, vertical tube 25 and verticaltube 32 are positioned in the same microwave reactor with the samesource of microwave energy.

FIG. 3 illustrates a process flow diagram for another embodiment of amicrowave regeneration system 50 for regenerating the saturated mediafrom vessels 15, 16, and 17 shown in FIG. 1. If the microwaveregeneration system is not collocated with the adsorbers 15, 16, 17, themedia saturated with siloxane, H₂S and water is transported to themicrowave regeneration facility in bulk or in portable adsorptionvessels. The contaminated media is stored in the media feed tank 52.From there, it is transported to the feed hopper 54 of the microwaveregeneration system.

The saturated media in the feed hopper 52 flows downward by gravitythrough a valve 56 to first lock hopper 58. Valve 56 prevents air(containing oxygen) from moving from feed hopper 54 to lock hopper 58.The media then flows into the first microwave reactor 60. In thisillustration, two parallel reactors 60 are shown to increase mediathroughput and provide redundancy. Details of an embodiment of microwavereactor 60 are shown in FIG. 4.

Returning to FIG. 3, media flows from the feed hopper 58, through firstmicrowave reactor 60, through a rotary feeder valve 62 that regulatesthe flow of media, and into lower lock hopper 64, through valve 66 andinto discharge hopper 64. From there, the media is transported to theregenerated media container 70.

A low volume inert sweep gas, such as Nitrogen 72, flows in tube 74 intolower lock hopper 64 and in a countercurrent direction through firstmicrowave reactor 60. Because media such as GAC is an excellentmicrowave absorbent, its temperature increases rapidly when exposed to amicrowave field. The rate of microwave solvent desorption in themicrowave field is an order of magnitude greater than the conventionalthermal desorption rate. The rapid heating and strong reducing characterof activated carbon in the regeneration system can also “crack” largerorganic molecules that are difficult to desorb using conventionalregeneration technologies. The resulting smaller molecules are then moreeasily removed from the carbon.

The siloxanes, H₂S, water and any large molecular weight hydrocarbons(VOCs), are desorbed rapidly from the media when exposed to microwaveenergy in first microwave reactor 60. Some siloxanes are de-polymerizedat the media surface by microwave energy to produce lower molecularweight siloxanes. As a result of the microwave energy, no siloxanecomponents will remain in the media. The desorbed gases are transportedout of the first microwave reactor 60 by the low volume sweep gas andinto tube 76. The power level of the microwave, the flow rate of media,and flow rate of sweep gas are controlled to remove the siloxanecomponents from media without decomposing the siloxanes into SiO₂. Theactive pores of the media are also regenerated by exposure to themicrowave energy in this step.

Next, desorbed siloxanes, water, H₂S and any VOCs are transported by thesweep gas in tube 76 to a second microwave reactor 80 with a source ofmicrowave energy and a silicon carbide (SiC) bed. A regulated source ofoxygen, such as air, is introduced to the sweep gas that flows to secondreactor 80 through tube 82. Siloxanes, H₂S and VOCs are oxidized in thesilicone carbide (SiC) bed by applying microwave energy to produce SO₂,water vapor, and CO₂ gas and particulate SiO₂. These gases produced inthe microwave oxidizer and the sweep gas entrain the particulate SiO₂and flow into tube 84 and enter into a first particulate filter 86 toremove granular SiO₂. In one embodiment, first filter 86 is a cleanableor vibrating filter. A cyclone may also be integrated into particulatefilter 86.

The remaining gases then flow through tube 88 and into a third microwavereactor 90 having a source of microwave energy and containing a reducingagent. GAC is an excellent microwave energy absorber and also strongreducing agent. It has been demonstrated that SO₂ reacts with GAC in thepresence of microwaves to produce elemental particulate sulfur andcarbon dioxide:SO₂+C→S+CO₂

GAC is consumed in this reaction and replenished from GAC feed hopper92.

The sweep gas and particulate sulfur flows out from the third microwavereactor 90 through tube 94 and enters into a second particulate filter96 that captures the elemental sulfur and remaining ash from consumingGAC. In one embodiment, particulate filter 96 is a cleanable orvibrating filter. A cyclone may be integrated into second particulatefilter 96. A gas cooler may also be integrated into filter 96 toincrease removal efficiency. The filtered sweep gas leaves second filter96 through tube 98 and is now clean gas consisting mostly of nitrogen,H₂O, and CO₂.

A part of the clean gas leaving the second filter is recycled backthrough tube 100 and a recycle compressor 102 to flow into tube 74. Anyexcess clean gas flows through tube 104 and through a final GAC adsorber106 to ensure that there are no fugitive contaminants in the clean gasbefore venting to the atmosphere at 108.

If halogenated hydrocarbons are present, the GAC in feed hopper 92 canbe impregnated with a metal oxide, such as NaOH. This will producehalogen salt particles, such as NaCl, that will be removed in secondparticulate filter 96. Alternatively, the clean gas in 104 can bebubbled through a wet scrubber or sparger containing a caustic solution,such as water and sodium hydroxide. The sodium in a sodium hydroxidesparger will react with the halogen in the vent gas to form a saltsolution that can be safely discarded.

The regenerated media that passed through first microwave reactor 60flows through rotary valve 62 and into lock hopper 64. From there themedia flows through valve 66 and into discharge hopper 68. The media isthen transported from discharge hopper 68 to the regenerated GAC storagetank 70. In one embodiment, a pneumatic blower 110 is used to transportthe GAC through the system. A particulate filter 112 is used to filterout carbon fines from handling and transporting the GAC. Alternatively,a mechanical transport system is used to transport the media throughmicrowave reactor 60.

The pores in the regenerated media are reactivated by the exposure tomicrowave energy in microwave reactor 60 and typically have the same orbetter adsorption capacity as new media. The regenerated media instorage tank 70 is transported back to the adsorption vessels 15, 16, 17shown in FIG. 1 to further clean the biogas. Depending on attrition dueto reaction or handling, new media may need to be added to regeneratedmedia storage tank 70.

It is estimated that this process can remove siloxane and H₂S frombiogas, and regenerate media at a cost less than replacing contaminatedmedia with new media.

The apparatus in FIG. 2 and FIG. 3 uses many conventional accessorieslike pumps, valves, gages, switches, controllers, etc. which arenecessary for safe operation of the process but are outside thenecessary components of the present invention and omitted for clarity.

Table 4 presents the material balance for an example regeneration systemfor digester gas depicted in Table 2. The material balance assumes amedia regeneration rate of 100 lb/hr. The contaminant loading ratio forH₂S, and siloxanes was determined from the material balance for the gaspretreatment system presented in Table 3. The material balance assumesthat the SiC microwave oxidizer operates with 15% excess air, and thatoxidation is complete. It is also assumed that the filters for removingSiO₂, sulfur, and ash are 100% efficient.

An energy balance on the system indicates that 8 kW of microwave energywill be required to desorb the contaminants in the first microwavereactor 60. The SiC oxidization reactor 80 and GAC reduction reactor 90each require about 3 kW of microwave power to heat the gas and solidsubstrates. There may be a requirement to cool the gas prior to enteringthe sulfur removal filter 96. The cooling load of 3 kW can be providedby an air cooling heat exchanger. Thus the energy required to treat 100pounds of media is about 14 kwh or about $1.45, a fraction of otherregeneration methods.

Landfill gas contains higher concentrations of heavy molecular weighthydrocarbons (molecular weight greater than 45) due to solvent disposal.Landfill gas is considered contaminated if heavy hydrocarbon loadingexceeds 50 ppmv. If the example is applied to landfill gas, the mediawill remove all heavy hydrocarbons from the gas. The presence of theheavy hydrocarbons requires 20% to 30% more media to clean a volume oflandfill gas as compared to a similar volume of digester gas. Thepresence of heavy hydrocarbons in the landfill gas is expected torequire about 20% to 30% more electrical energy to regenerate theadditional media and destroy the additional contaminants.

The example multi stage media adsorption system is expected to remove atleast 90% of the siloxanes and H₂S from the biogas. The microwavetreatment system is expected to remove over 95% of the siloxanes and H₂Sfrom the media. There is expected to be a small (1-3%) attrition ofmedia from handling and regeneration that will be replaced with newmedia.

FIG. 4 is a schematic view of a microwave reactor 60 shown in FIG. 3. Atube 120, which is transparent to microwave energy, is positioned inmicrowave cavity 122. Microwave cavity 122 will contain microwave energyand, in this example, is shaped similar to a rural mailbox. In oneembodiment, tube 120 is quartz glass and about 4 inches in diameter toprovide good microwave penetration to media in the center of the tube.

A rectangular wave guide 124 is attached to microwave cavity 122 andcommunicated energy through a row of slots 126. The width of the slotand spacing is optimized for microwave energy. A source of microwaveenergy 128 is attached to waveguide 124.

Contaminated media enters tube 120 at media inlet 132 and flow downwardthrough the microwave field and exits at media outlet 134. Sweep gasenters at the bottom of the reactor at gas inlet 136 and flows upwardthrough tube 120 and out at gas outlet 138. When exposed to microwaveenergy, gaseous contaminants are quickly desorbed from the media in tube120 and removed in the sweep gas.

In another embodiment of the invention, microwave cavity 122 iswater-cooled. In a further embodiment, microwave cavity 122 isair-cooled.

Third microwave reactor 90, shown in FIG. 3, is of similar design.Second microwave reactor 80 is of similar design except the oxidationcatalyst, such as Silicon Carbide, in tube 120 is fixed.

FIG. 5 is a schematic view of another embodiment of a siloxane andhydrogen sulfide removal process 150 with a media adsorber integratedinto the system. In this process, some of the BTU content incontaminants such as H₂S, Siloxane and hydrocarbons is recovered andrecycled into the biogas stream and there are no direct atmosphericemissions. Biogas produced in landfills may have up to about 5-10%hydrocarbons having a molecular weight above 45 that are derivedprimarily from solvents disposed in the landfill. Many of these heavyhydrocarbons contain halogens. Burning these heavy hydrocarbons in anengine, turbine or flare can cause harmful deposits or undesirable airpollution. Media such as GAC readily adsorb these heavy hydrocarbonsfrom biogas.

Schematically, media adsorber 152 is a two-stage adsorber with primarystage 154 and secondary stage 156. When the media in primary stage 154,such as GAC, has been saturated with siloxane and H₂S, or breakthroughof a contaminant is detected or calculated, it is transported to feedhopper 54. Media in secondary stage 156 moves down by gravity andreplaces the media removed from primary stage 154. Biogas from adigester or landfill enters media adsorber 152 at inlet 158 in primarystage 154, flows through secondary stage 156, and exits GAC adsorber 152at outlet 160. In one mode of this embodiment, media adsorber 152 is atwo stage radial adsorber with homogeneous media. In another embodiment,media adsorber 152 is a plurality of media adsorbers connected inseries. The capacity of a typical commercial media adsorber ranges fromabout 100 pounds to about 3,000 pounds of media such as GAC.

Biogas at inlet 158 entering media adsorber 152 has been pretreated forexcess water and high levels of H₂S as previously described in FIG. 1.Note, however, that treated biogas typically still contains water vapor.The temperature of the biogas or the adsorber may also be slightlywarmed to prevent water condensation in the media adsorber.

The contaminated media from primary stage 154 is transported throughconduit 162 to the feed hopper 54 of the microwave regeneration system.

The contaminated media in feed hopper 54 flows downward by gravitythrough a valve 56 to first lock hopper 58 which seals air from movingfrom feed hopper 54 to first lock hopper 58. The media then flows intothe first microwave reactor 60. In this illustration, two parallelreactors 60 are shown to increase media throughput. Media flows from thelock hopper 58, through the tube in the microwave reactor 60, andthrough a rotary feeder valve 62 that regulates the flow of media intolock hopper 64. The media then travels through valve 66 and intodischarge hopper 64. From there, the media is transported throughconduit 164 back to the secondary stage 156 of media adsorber 152. If amechanical media transport system that does not introduce air into thesystem is used, feed hopper 54, lock hopper 58, lock hopper 64 anddischarge hopper 68 may be eliminated from the system. Valves 56 and 66would be used to keep biogas out of microwave reactor 60.

In a further embodiment, two or more adsorbers 152 containing differentpore size media are used in series in the biogas stream. One microwavereactor 60 is dedicated to each adsorber to keep the different mediaseparate through the regeneration process.

A low volume inert sweep gas, such as Nitrogen 72, flows in tube 74 intolower lock hopper 64 and in a countercurrent direction through firstmicrowave reactor 60. The siloxanes and H₂S, as well as water and highmolecular weight hydrocarbons are desorbed rapidly from the media whenexposed to microwaves in first microwave reactor 60. Some siloxanes maybe de-polymerized at the carbon surface by microwave energy to producelower molecular weight siloxanes before desorbing from the media. As aresult of the exposure to microwave energy, no siloxane or hydrocarboncomponents will remain in the media. The power level of the microwave,the flow rate of media, and flow rate of sweep gas are controlled toremove the siloxane components from media but not decompose thesiloxanes into SiO₂. The active pores of the media are also regeneratedby exposure to the microwave energy in this step.

Siloxanes, H₂S, water vapor and any VOCs desorbed from the media aretransported out of microwave reactor 60 by the sweep gas in tube 170 toa reduction microwave reactor 172 containing a reducing agent such asGAC.

The GAC is an excellent microwave absorbent and also reducing agent forhydrocarbons. Consequently, the siloxanes will react with water vapor inthe presence of the GAC bed when exposed to microwave energy to producemethane and SiO₂ as shown in two examples below:Si₂(CH₃)₆O(Hexamethyldisiloxane)+3H₂O→6CH₄+2SiO₂Si₅(CH₃)₁₀O₅(Decamethylcyclopentasiloxane)+5H₂O→10CH₄+5SiO₂

Other siloxanes species and siloxane containing hydrocarbons react withwater vapor when exposed to microwave energy in the reduction reactorfollowing a similar reaction path as shown above. If the media fromadsorber 152 is unusually dry, water can be introduced into sweep gas in170 and into reduction reactor 172.

Any high molecular weight VOCs containing Oxygen will decompose to formCO₂ and lower molecular weight hydrocarbons such as methane in reductionreactor 172. Other hydrocarbons will crack to lower molecular weighthydrocarbons, such as methane, and release hydrogen in the presence ofmicrowave energy. The reactants then flow through tube 176 toparticulate filter 178. The silicon oxide (SiO₂), in the form of aparticulate, is removed, and the remaining gasses (CH₄, H₂O, CO₂, etc.)exit filter 178 and flow through tube 180 and into primary stage 154 ofmedia adsorber 152. Methane and CO₂ are not readily adsorbed by themedia in media adsorber 152 and exit with the biogas at outlet 160. Anyremaining hydrocarbons with molecular weight higher than about 45 willbecome adsorbed in media in adsorber 152 and reintroduced with the mediainto microwave reactor 60.

If halogenated hydrocarbons are present, the GAC in the reductionreactor 172 can be impregnated with a metal oxide such as NaOH. Thesodium will react with the halogens to produce halogen salts, such asNaCl, that will be removed as a particulate in filter 178. Ash fromreacted GAC is also removed by filter 178. In one embodiment,particulate filter 178 is a cleanable or vibrating filter. A cyclone maybe integrated with filter 178. A gas cooler may also be integrated intofilter 178 to increase efficiency.

If high molecular weight siloxanes are only partially decomposed andremain in a gaseous state, they will pass through filter 178 and throughtube 180 to media adsorber 152. Here they will be adsorbed again in themedia and reintroduced into microwave reactor 60. Eventually, they willbe decomposed to low molecular weight hydrocarbons, such as methane, andSiO₂. The SiO₂ particulates will be filtered from the system in filter178 and the methane will pass through the media in media adsorber 152and exit outlet 158 with the biogas.

If hydrogen sulfide is present in the biogas, it will be adsorbed in themedia in adsorber 152 and desorbed from the media in microwave reactor60 in the presence of microwave energy and removed in sweep gas 170.Biogas from a landfill may have 1-2% oxygen. Some media, such as GAC,will adsorb a fraction of this oxygen. If oxygen is present in the sweepgas or in the GAC, hydrogen sulfide will preferentially form water inreactor 172:H₂S+½O₂→H₂O+S

When all oxygen is removed or consumed, hydrogen sulfide is dissociatedinto hydrogen and elemental sulfur in the presence of the reducing agentwhen exposed to microwave energy in reduction reactor 172 as shownbelow:H₂S+½O₂→H₂+S

The reactants flow through tube 176 and elemental sulfur in particulateform is removed by filter 178. Hydrogen gas flows through tube 180 tocombine with biogas in media adsorber 152 and will pass through themedia to outlet 160. In one mode, some of the reacted gasses fromreduction reactor 172 flow through tube 100 to compressor 102 and arerecycled through the microwave reactor through tube 74. Note that allexcess gas from microwave reduction reactor 172 is reintroduced into themedia adsorber 152 with the biogas. There are no atmospheric emissionsfrom this system. In one mode of this process, the reacted gasses arechilled and excess water is condensed and removed from the gas in tube180 before returning to media adsorber 152.

In one embodiment of the invention, the media in media adsorber 152 isGAC. In another embodiment of the invention, the media in media adsorber152 is a molecular sieve.

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more.” All structural, chemical, and functionalequivalents to the elements of the above-described preferred embodimentthat are known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe present claims. Moreover, it is not necessary for a device or methodto address each and every problem sought to be solved by the presentinvention, for it to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed under the provisions of 35U.S.C. 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for.”

TABLE 1 Common Volatile Siloxanes Vapor Water Press. Boiling Solub. mmHgPoint, (mg/L), Name Formula MW at 77° F. Abb. ° F. 77° F.Hexamethylcyclotrisiloxane C₆H₁₈O₃Si₃ 222 10 D₃ 275 1.56Octamethylcyclotetrasiloxane C₈H₂₄O₄Si₄ 297 1.3 D₄ 348 0.056Decamethylcyclopentasiloxane C₁₀H₃₀O₅Si₅ 371 0.4 D₅ 412 0.17Dodecamethylcyclohexasiloxane C₁₂H₃₆O₆Si₆ 445 0.02 D₆ 473 0.005Hexamethyldisiloxane C₆H₁₈Si₂O 162 31 L₂, 224 0.93 MMOctamethyltrisiloxane C₈H₂₄Si₃O₂ 236 3.9 L₂, 0.035 MDMDecamethyltetrasiloxane C₁₀H₃₀Si₄O₃ 310 0.55 L₃, MD₂MDodecamethylpentasiloxane C₁₂H₃₆Si₅O₄ 384 0.07 L₄, MD₂M

TABLE 2 Example Material Balance for biogas pretreatment system.Pressure Temp CH₄ CO₂ H₂O H₂S C₈H₂₄O₄Si₄ C₁₀H₃₀O₅Si₅ Stream (psig) (F.)(gmol/hr) (gmol/hr) (gmol/hr) (gmol/hr) (gmol/hr) (gmol/hr) Biogas FeedStream 7 45 4046 3237 47 0.4 2.13E−03 2.42E−02 Exit Media #1 6.5 55 40463237 23 0.06 2.13E−03 2.42E−02 Exit Media #2 6 55 4046 3237 23 5.52E−021.91E−03 1.21E−02 Exit Media #3 5.5 55 4046 3237 23 5.52E−02 9.56E−051.21E−03 Exit Media #4 5 55 4046 3237 23 5.52E−04 9.56E−06 1.21E−04

TABLE 3 Example Media Performance Media #1 Media #2 Media #3 Media #4Media Molecular AFT “DD” AFT “DM” AFT “MD” Sieve H2S removal 85% 0% 0%90% Efficiency H2O removal 50% 0% 0%  0% Efficiency D4 removal  0% 10% 95%  90% Efficiency D5 removal  0% 50%  90%  90% Efficiency EstimatedOverall 25% 5% 5%  5% Adsorption Capacity lbs adsorbent/hr 3.78 0.2000.202 0.1007

TABLE 4 Material Balance for Example Carbon Regeneration System H₂S SiO₂Temp Pres GAC (lb/ N₂ O₂ SO₂ S CO₂ H₂O Ash C₈H₂₄O₄Si₄ C₁₀H₃₀O₅Si₅ (lb/(F.) (psig) (lb/hr) hr) (lb/hr) (lb/hr) (lb/hr) (lb/hr) (lb/hr) (lb/hr)(lb/hr) (lb/hr) (lb/hr) hr) Carbon Inlet 70.0 0 100 0.642 0 0 0 0 0 21.50 3.23E−02 4.59E−01 0 Regenerated 250 0.500 99.0 0 0 0 0 0 0 0 0 0 0 0Carbon Recycled 95 10.0 0 0 9.36 0 0 0 0.843 1.87 0 0 0 0 Sweep GasSaturated 250 7.00 0 0.642 9.36 0 0 0 0.843 23.4 0 3.23E−02 4.59E−01 0Sweep Gas Oxidation 70.0 7.00 0 0 8.96 2.38 0 0 0 0 0 0 0 0 Air Oxidized500 6.00 0 0 18.3 0.522 1.21 0 1.43 24.1 0 0 0 0.397 Sweep Gas SiO2Filter 500 4.00 0 0 0 0 0 0 0 0 0 0 0 0.397 Reacted 500 3.00 0 0 18.3 00 0.604 2.97 24.1 0.0676 0 0 0 Sweep Gas Ash-Free 150 2.00 0 0 18.3 0 00 2.97 4.6 0 0 0 0 Sweep Gas Ash Outlet 150 0 0 0 0 0 0 0 0 20 0.0676 00 0 Reactor 70.0 0 0.640 0 0 0 0 0 0 0 0 0 0 0 Makeup Carbon Sweep 1252.00 0 0 8.96 0 0 0 2.13 1.8 0 0 0 0 Gas Purge Vent to 125 0 0 0 8.96 00 0 2.13 1.8 0 0 0 0 Atmosphere Nitrogen 32.0 15.0 0 0 0 0 0 0 0 0 0 0 00 Supply

1. A method for removing siloxane from adsorbent media and decomposingthe siloxane, comprising: providing a first microwave reactor having asource of microwave energy; wherein said first microwave reactor isconfigured to receive adsorbent media containing siloxane; providing asecond microwave reactor having a source of microwave energy fluidlyconnected to said first microwave reactor; providing a sweep gas flowingthrough said first microwave reactor and said second microwave reactor;positioning adsorbent media containing siloxane in said first microwavereactor; applying microwave energy to said first microwave reactor toremove siloxane from said adsorbent media; transporting siloxane to saidsecond reactor by said sweep gas; introducing water into said secondreactor; and applying microwave energy to said second microwave reactorto form silicon dioxide from siloxane.
 2. A method as recited in claim1, wherein said second microwave reactor contains a reducing agent.
 3. Amethod as recited in claim 1: wherein the adsorbent media containshydrogen sulfide; applying microwave energy to said first microwavereactor to remove hydrogen sulfide from said adsorbent media;transporting hydrogen sulfide in said sweep gas from said firstmicrowave reactor to said second microwave reactor; and formingelemental sulfur from hydrogen sulfide when exposed to microwave energyin said second microwave reactor.
 4. A method as recited in claim 1:providing a first particulate filter fluidly connected to said secondmicrowave reactor; providing an adsorber fluidly connected to said firstmicrowave reactor; positioning said adsorbent media in said adsorber;flowing biogas containing siloxane through said adsorbent media in saidadsorber; adsorbing siloxane from said biogas into said adsorbent media;transporting said adsorbent media containing siloxane to said firstmicrowave reactor; flowing said sweep gas from second microwave reactor,through said first filter and into said adsorbent media in saidadsorber; and removing silicon dioxide from said sweep gas in said firstfilter.
 5. A method as recited in claim 4: wherein said biogas containsheavy hydrocarbons having molecular weight greater than forty five;adsorbing heavy hydrocarbons from said biogas into said adsorbent media;transporting said adsorbent media to said first microwave reactor;applying microwave energy to said first microwave reactor to removeheavy hydrocarbons from said adsorbent media; transporting heavyhydrocarbons in said sweep gas from said first microwave reactor to saidsecond microwave reactor; forming methane from heavy hydrocarbons whenexposed to microwave energy in said second microwave reactor; andtransporting methane from said second microwave reactor to said adsorberin said sweep gas.
 6. A method as recited in claim 1: providing anoxidizing catalyst in said second microwave reactor; providing a sourceof oxygen to said second microwave reactor; providing a firstparticulate filter fluidly connected to said second microwave reactor;providing a third microwave reactor having a source of microwave energy;wherein said third microwave reactor is fluidly connected to said secondmicrowave reactor; providing a reducing agent in said third microwavereactor; providing a second particulate filter fluidly connected to saidthird microwave reactor; wherein said sweep gas flows from said secondmicrowave reactor through said first filter, through said thirdmicrowave reactor and through said second filter; wherein said adsorbentmedia contains hydrogen sulfide; applying microwave energy to said firstmicrowave reactor to remove hydrogen sulfide from said adsorbent media;transporting siloxane and hydrogen sulfide in said sweep gas from saidfirst microwave reactor to said second microwave reactor; formingsilicon dioxide from siloxane when exposed to microwave energy in saidsecond microwave reactor; removing silicon dioxide in said first filter;forming sulfur from hydrogen sulfide when exposed to microwave energy insaid second microwave reactor; transporting sulfur dioxide in said sweepgas from said second microwave reactor to said third microwave reactor;forming sulfur from sulfur dioxide when exposed to microwave energy insaid third microwave reactor; and removing sulfur from said sweep gas insaid second filter.
 7. A method as recited in claim 6: wherein saidadsorbent media is carbon; wherein said oxidizing catalyst is siliconcarbide; and wherein said reducing agent is carbon.
 8. A method asrecited in claim 6, further comprising providing a metal oxide in saidreducing agent.